Mobile IP (RFC 2002) has been suggested in the Internet Engineering Task Force (IETF), as a method for maintaining communication without interrupting the session by a mobile node MN, when a terminal having an IP address moves across a subnet (see “RFC 2002: IP Mobility Support” by C. Perkins, IETF Internet Draft, 1996/10, pages 8 to 11).
This concept is explained in detail in “Detailed Mobile IP” (see “Detailed Mobile IP” by James D. Solomon, Pearson Education Japan, Jul. 27, 1998, pages 57 to 77).
FIG. 13 illustrates an example of communication using the Mobile IP. Reference sign 1a denotes an IP subnetwork, 1b denotes an IP subnetwork belonging to a subnet different from the IP subnetwork 1a, 2 denotes a mobile node MN mounted with the Mobile IP, 3a and 3b denote access points accommodating the mobile node, 4 denotes a Mobile IP foreign agent (FA), 5 denotes a home agent (HA) controlling the position of the mobile node 2 as the IP address of the FA, and 6 denotes a communication partner node (CN) communicating with the mobile node.
In the Mobile IP, the HA 5 is arranged in the home domain of the mobile node (MN) 2, and an IP address belonging to this home domain (hereinafter, “home address”) is allocated to the mobile node (MN) 2 in a fixed manner. The FA address at which the mobile node (MN) 2 is present is controlled by the HA 5 as a position of the mobile node (MN) 2. Therefore, the mobile node (MN) 2 registers in the HA a new FA address (referred to as an FA care-of address) as an address to which the mobile node (MN) 2 moves, via the new FA, at a timing of detecting that the mobile node (MN) 2 moves from under a supervision of the old FA to under the new FA.
When the CN 6 transmits an IP packet to the mobile node (MN) 2, the HA 5 controlling the FA care-of address of the mobile node (MN) 2 captures the packet addressed to the home address of the mobile node (MN) 2, and transfers the captured IP packet to the registered FA 4 through an IP tunnel. The transfer through the IP tunnel stands for encapsulating the IP packet received by the HA with an IP header having:                a sender address: IP address of the HA 5, and        a destination address: FA care-of address,and transferring it to the FA. Upon reception of the encapsulated IP packet, the FA 4 takes out the original IP packet therefrom, and transfers it to the mobile node (MN) 2. By this mechanism, movement transparency of the mobile node (MN) 2 is realized in the Mobile IP.        
When the mobile node (MN) 2 obtains the IP address in the subnet at the moving destination, the mobile node (MN) 2 can directly register this IP address (referred to as a collocated care-of address) as an address to which the mobile node (MN) 2 moves, in the HA 5. In this case, the IP tunnel is set not between the FA 4 and the HA 5, but between the mobile node (MN) 2 and the HA 5.
The IP packet transmission from the mobile node (MN) 2 to the CN 6 is performed by transmission via a normal routing or by transmission via the IP tunnel.
In the conventional configuration, since the FA and the HA have functions of signaling processing of the Mobile IP and IP tunnel transfer processing of a user packet, the load concentrates on these parts.
Generally, the next-generation network has a network configuration in which a control plane for performing signaling processing, and a user plane for performing user packet processing are separately mounted. In such a distributed network, independent development of the signaling processing and the user packet processing, realization of scalability of each plane, realization of load balancing on each plane, low cost of the general-purpose user packet processor, and high performance and low cost by realizing the signaling processing on a general-purpose computer are expected. However, the Mobile IP does not assume such a distributed network, and hence expansion to the next-generation network is difficult.
Particularly, in the Mobile IP, the FA assumes a message informing the own address, and the mobile node MN regards the sender IP/MAC addresses in this message as the FA, to perform position registration signaling, and also regards it as a default router to transmit the user data. This indicates that one IP address is used for both signaling and user packet processing, and hence the control plane and the user plane cannot be separated.
Further, it is not specified how the address information informed by the position registration from the mobile node MN to the HA is handled, when the control plane and the user plane are separated.
With regard to the procedure when the mobile node MN does not know the address of the HA control server, it is not specified how it is changed, when the control plane and the user plane are separated.